1. Field of the Invention
The present invention relates to improvements of a positive electrode active material for a nonaqueous electrolyte secondary battery, a method for making the positive electrode active material, etc. The present invention relates to a positive electrode active material for a nonaqueous electrolyte secondary battery that can yield high reliability even when the battery features high capacity, a method for making the positive electrode active material, etc.
2. Description of Related Art
Reduction of the size and weight of mobile information terminals such as cellular phones, notebook computers, and personal digital assistants (PDAs) has progressed rapidly in recent years, and batteries used for driving such terminals are required to achieve ever higher capacity. Of the secondary batteries that can meet such demand, nonaqueous secondary batteries that use an alloy, a carbon material, or the like that can occlude and release lithium ions, as a negative electrode active material and a lithium transition metal complex oxide as a positive electrode active material are drawing much attention as a battery that has a high energy density.
The capacity of nonaqueous electrolyte secondary batteries has been increased by reducing the thickness of components not related to the capacity, such as a battery can, a separator, and a collector (aluminum foil or copper foil), and by increasing the amount of the active material packed in the electrode (improvement of electrode packing density). However, even if those approaches for increasing the capacity are employed, the capacity of the nonaqueous electrolyte secondary batteries cannot be sufficiently increased. Another conceivable approach to increasing the capacity and energy density is to raise the charge cut-off voltage. However, if the charge final voltage is raised, the positive electrode active material may be deteriorated and the electrolyte may be decomposed by oxidation, resulting in lowering of the battery characteristics.
To address this issue, as described below, there has been proposed a technique for suppressing the reaction between the positive electrode and the electrolyte in a charged state.
A positive electrode active material for a lithium secondary battery has been proposed in Japanese Unexamined Patent Application Publication No. 2009-218217 (Patent Document 1). This positive electrode active material includes a core that contains at least one lithium compound and a surface-treated layer on the core and exhibits good electrochemical characteristics such as lifetime, discharge potential, and electric energy characteristic since the surface-treated layer contains at least one coating material selected from a hydroxide, an oxyhydroxide, an oxycarbonate, and a hydroxycarbonate of a coating element which is at least one metal selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, and a mixture thereof.
However, this technique does not sufficiently suppress the reaction between the positive electrode and the electrolyte in a high-temperature high-voltage charge state. When a battery is stored and retained in this state, formation of films and gassing caused by decomposition of the electrolyte cannot be sufficiently suppressed.